ANALYSIS OF SPATIAL AND TEMPORAL ORGANIZATION IN A SALMONID BROOK IN RELATION TO PHYSICAL FACTORS AND MACROPHYTIC VEGETATION

Four compartments of a salmonid brook were studied in consecutive sect ions either at sector scale or at facies scale: physical features, wat er quality, macrophytes and salmonids. The most important factor in th e spatial and temporal organisation was the longitudinal zonation asse ssed by the four compartments. It was mainly shown by a gradient of th e length of the morphodynamic sequence and of the facies width, of the pH and the conductivity and of macrophyte cover. It induced a colonis ation by older trout in the downstream part of the brook where growth was better and shelter given by the bankside vegetation was more abund ant. There were also greater densities of other age-classes upstream, with a correlation between salmon and rock shelter. Furthermore, the s econd factor corresponded to the flow characteristics and a granulomet ry gradient. The third factor corresponded chiefly to lighting, salmon numbers and densities. Salmonids showed a temporal stability from spr ing to autumn, and an opposition between growth and density. Six clust ers of sectors defined functional entities. With regard to the 3-dimen sional structure of the brook, the special pattern of each compartment shows its contribution to the functioning of the whole. Taking into a ccount the scale of the study, the coexistence between trout and salmo n did not show a competition between the two species, but rather a spa tial partition, thanks to the heterogenity inside each sector. We conc luded on the necessity of studying sympatry at the facies scale to ass ess the role of habitat factors as regulators of carrying capacity.